1. Field of the Invention
The invention relates in general to an image forming device for a photocopying apparatus and, in particular, to a new and useful image forming device for copying apparatus of the type which are capable of forming duplicates of a size different from the size of an original.
2. Description of the Prior Art
In copying apparatus of the variable duplicate size type, particularly in copying apparatus of the type generally used which is adapted to produce a duplicate of a reduced size from an original, the light flux passing through the optical system for exposing an image forming surface to a optical image of an original tends to become high in intensity within a slit mounted in the vicinity of the image forming surface. The result of this is that the image forming surface tends to be overexposed.
When an original is copied to produce a duplicate which is a different size than the original, a great change is generally produced in the volume of light illuminating a photosensitive member per unit area depending on the rate of magnification or reduction. The intensity of illumination E (1x) of an image at a given magnification can be expressed by the following formula ##EQU1## WHERE .beta. IS THE DEGREE IN CHANGE OF THE SIZE OF THE DUPLICATE, L (cd/m.sup.2) is the luminocity of a unit area of the original disposed on the optical axis and arranged perpendicularly thereto, .tau. is the transmission factor of the lens system, f (m) is the focal distance of the lens, and .phi. (m) is the effective aperture of the lens.
Since .tau., L, .phi., and f are constant in copying apparatus of common use having a single lens system, the intensity of illumination of the image E can be expressed as follows: ##EQU2##
In copying apparatus wherein a slit exposing system is used, the width of the slit relative to the image forming surface is restricted to a value below a certain level due to the conditions placed by the charger and developer or the condition placed by the diaphragm plate. Assume that the width of the slit relative to the image forming surface is constant W.sub.1 (m) without the conditions placed by the throttle plate and the like being unchanged by the rate of change of the size of the duplicate, the exposure I.sub.1 (1x.multidot.sec) of a photosensitive member at the duplicate size change rate .beta. can be expressed as the product of the intensity of illumination of the image and the time during which the photosensitive member is exposed. ##EQU3## where v (m/sec) is the velocity of the photosensitive member. As compared with the exposure obtained when a duplicate of the same size as an original is produced, the exposure increases up to about 180 percent when the rate of reduction is 1/2 and decreases to 65 percent when the rate of enlargement is 1.5.
Due to the fact that a great change in exposure is caused by the duplicate size change in producing a copy, the photosensitive member becomes either overexposed or underexposed. This is not acceptable in copying apparatus which adopt an electrophotographic system or any other system.
To solve this problem, proposals have hitherto been made to use a method which provides light volume control means for varying the light volume in accordance with the duplicate size change rate, or to use a method which provides means on the surface of an original for regulating the width of the slit to a predetermined level to thereby keep the exposure constant.
As a method to provide exposure control means which can vary the volume of light depending on the duplicate size change rate, a lens having a variable diaphragm may be used and the amount of light entering the lens may be varied in conjunction with the movement of the lens each time the duplicate size change rate is varied; a movable diaphragm plate may be mounted in the vicinity of the image forming surface and the diaphragm plate may have its shape changed or the diaphragm plate may be moved in conformity with the duplicate size change rate so as to thereby change the width of the slit; or the volume of emanated light, the efficiency of illumination, the efficiency of emanating light, and the time during which light is emanated may be varied. All of these methods require the use of a special device for compensating for a change in the exposure, so that they have the disadvantages of rendering the apparatus complex in construction, increasing cost and lowering the reliability of the performance of the apparatus. Moreover, it is impossible to effectively compensate for a change in exposure by these methods, and difficulties are encountered in placing the exposure within the tolerance limits.
The method for limiting the width of a slit to a predetermined level can achieve marked results in compensating for a change in exposure. However, in copying apparatus of the type in which the original is placed in an original placing station to perform slit exposing of the image forming surface to the original, it is difficult in actual practice to restrict the width of the slit at the surface of the original because presence of the original placing glass surface or glass placing support deck interferes with this arrangement. Moreover, when this method is used, there is a change in the volume of light when a duplicate of a reduced size is produced, so that the method does not lend itself to use with copying apparatus in which a process sensitive to a change in the light volume is used.